It's a mushroom from japan. It's considered to have healing properties.

Webster: a dark Oriental mushroom (Lentinus edodes of the family Agaricaceae) widely cultivated especially on woods of the beech family for its edible flavorful cap.

An extremely flavorful mushroom which can be found in many markets. Shitake (Japanese for oak mushroom) mushrooms can be found dry or fresh. Dried Shitake mushrooms have a very strong flavor and thus used in smaller quantities. They are also soaked before they are cooked.

Shitake mushrooms are grown on oak logs, hickory logs, or sweet gum logs.

If you're going to buy Shitake mushrooms, make sure that the mushrooms were log grown.